What causes bay windows to subside? 

Bay windows subside for the same reasons an entire property may subside, whether they’re part of the original house or were retrofitted on at a later time. Subsidence occurs when a structure begins to sink or shift, resulting in structural damage such as cracked walls or sloping floors. Here are some of the most common causes:  

Poor foundation construction 

One of the most common causes of bay window subsidence is poor foundations. There is a historical misconception that bay windows, similar to conservatories and extensions, were lighter than the main part of the house, which meant they were built with lighter foundations. Over time, the foundation shifts because it cannot support the weight of the bay window above, causing it to move.  

Soil shrinkage or erosion 

During periods of prolonged hot weather or droughts, soils (particularly clay soils) can shrink, causing the foundations beneath the bay window to shift or sink.  

Soil erosion can also cause problems. Heavy, intense rainfall can wash away soil particles, which weaken the ground and cause it to shift unevenly. This could be particularly concerning if your bay window is close to a riverbed.  

Tree roots 

If you have trees near your home, then their roots can pose a risk of subsidence. Tree roots spread out in search of moisture and can branch out beneath your property foundations. This can be more problematic if your house sits on clay soil because the roots will draw out significant amounts of moisture, causing the soil to shrink. As a result, the foundations become unstable. 

Nearby traffic  

Roads are becoming busier. High volumes of traffic on nearby roads can create underground vibrations that affect the soil over time and weaken ground conditions. These vibrations can be especially problematic if the ground is already susceptible. For example, if the ground has become oversaturated following a burst water pipe.  

The weight of cars, EVs and heavier vehicles such as vans and lorries can also compress soils, causing the ground to sink.  

Historic mining activities or fracking 

Old mining tunnels can collapse if their ageing supporting pillars fail or decay, causing the surface of the ground to sink. If your property sits above a historic mining tunnel, you should consider arranging for a mining search report.  

Similarly, fracking activity can cause significant ground disturbance, which can shift subsoils. The fracking process involves drilling underground and injecting water, sand and chemicals into the rock, forcing gas and oil to escape. This high-pressure activity can disturb surrounding soils, potentially leading to subsidence or settlement. 

Signs of bay window subsidence 

So, what does bay window subsidence look like? The signs are similar to regular house subsidence, except that it’s more localised to the bay window area. Here’s what to look out for: 

• Cracks that form between the bay window and the main house

• Cracks around the windows

• Wide, diagonal, zig-zag cracks

• The window frame is leaning significantly to one side or coming away from the main structure

• Sloping or sinking floor where the bay window sits

Other signs of bay window subsidence include uneven window alignment and difficulty in opening or closing windows. 

How to fix bay window subsidence 

If you’ve spotted any signs of bay window subsidence, your best course of action is to seek professional advice. You can book a subsidence survey with Mainmark UK, which will identify whether your bay window is experiencing movement, what’s causing it and how to treat it.  

Mainmark UK offers a range of solutions to remedy bay window subsidence and ground movement issues, including non-invasive techniques such as resin injection and JOG computer-controlled grouting. Teretek® resin injection involves injecting expanding resins into small holes in the ground to lift and re-level structures. As the name suggests, JOG computer-controlled grouting improves soils and lifts foundations using a cement grout that’s injected under computer control. Both modern techniques provide an efficient solution with minimal disruption, offering you peace of mind and the ability to remain in your property while work is carried out.  

Taking action early 

Identifying bay window subsidence can be unsettling, but the good news is that if you act early, you can prevent further damage and protect the value of your property. With the right treatment method, you can address the issue quickly so that your bay window remains structurally sound for many years. 

If you have any concerns about bay window subsidence, contact Mainmark UK today and arrange for a free in-person site survey.